System for anchoring ships

ABSTRACT

System for mooring ships, in particular for operations in connection with oil and gas activity, whereby a ship is equipped with a mooring device at its bow portion. There is included an anchor device located at the seabed, as well as at least one anchoring line adapted to connect the anchor device to the mooring device on the ship. The anchor device is an essentially permanent anchor device, preferably in the form of a suction anchor, gravitation anchor or pile anchor. The anchor device is provided with a swivel device for the anchoring line. A buoyant body is attached to a middle portion of the anchoring line.

This invention relates to a system for mooring ships, in particular foroperations in connection with oil and gas activity, whereby the shipsconcerned are equipped with mooring means at their bow part, and wherethere is included anchor means located at the seabed, as well as atleast one achoring line adapted to connect the anchor means to themooring means on the ship.

In offshore oil and gas activity there is often the question of veryimportant operations that can be difficult under certain conditions, andwhereby there is usually involved transfer of fluids either between twoships, of which one can be moored, or between a pipeline connected tothe anchor means at the seabed and a moored ship. Under varying anddifficult conditions, whereby wind, waves and ocean current haveinfluence, great stresses and forces can occur during such mooring andcarrying out of these operations. Such stresses in the first place canlead to interruption of the operations and in the worst case can lead towrecking and e.g. uncontrolled oil discharge. It is obvious that thesystem according to the invention depending on the circumstances, canalso be utilized for other types of operations at sea, than inconnection with oil and gas activity.

On the background of mooring systems being known for correspondingpurposes, this invention involves novel and specific features as statedmore closely in the claims.

Among the advantages obtained by means of the invention, it isemphasized in particular that the challenging operations mentioned, canbe carried out under difficult conditions with higher security andreliability in most situations, compared to previously known methods andsystems. In this connection it is to be noted in particular that thesystem according to the invention makes possible a type of elasticity orflexibility in the mooring and possibly the fluid transfer, thatinvolves adaptation of the whole system according to the stresses andforces occurring during the operations to be performed.

In the following description the invention will be explained moreclosely with reference to the drawings, in which:

FIG. 1 schematically shows a first embodiment of the system according tothe invention,

FIG. 2 more in detail and elevation shows an anchor with associatedswivel means, which can be included in a system according to theinvention, and

FIG. 3 shows the same as FIG. 2 in front elevation.

In FIG. 1 of the drawings the seabed is indicated at 1 and the seasurface at 2, as well as substantially the whole system according to theinvention and the total arrangement involved in a mooring situation withassociated operations. There is here in the first place the question ofa ship 10, usually a tanker, an anchor 3 at the seabed 1 and ananchoring line with two parts 6 and 8 being at a middle portion providedwith a buoyant body 7, also denoted line buoy. In the usual manner theship 10 is equipped with mooring means 11 at the bow, without anydetails being shown more closely at this point.

The system according to the invention as described so far, is sufficientfor the desired mooring of the ship 10, and in this connection involvesadvantages as already mentioned in the introduction above. An importantfeature of the mooring system is the line buoy 7, which is preferablylocated at or connected to a middle portion of the total anchoring line6, 8. It is obvious that buoy 7 does not need to be exactly at themiddle of the total line length, but in order that the desired effect beobtained, it is and advantage that the buoy is positioned at a gooddistance both from the lower end of anchoring line 6 at anchor means 3,and from the upper end of anchoring line 8 at mooring means 11.

The dimensions of buoy 7 are chosen so that under most conditions orstresses a quite significant angle difference between the adjacentportions of line parts 6 and 8 is established. Thus line part 6 willnormally extend upwards from anchor 3 at a clearly smaller angle inrelation to the vertical, than the angle at which line part 8 runs outfrom buoy 7. When the ship 10 is strongly affected by wind, waves orocean currents, the whole anchoring line 6, 8 may be tightened more thanshown e.g. in FIG. 1, so that buoy 7 is pulled deeper into the water andthe angle between line parts 6 and B can approach more or less 180°. Asan opposite extreme when a minimum of mooring forces are acting, buoy 7may float to the sea surface 2, if the length of line part 6 is largerthan the water depth.

The latter situation will be most likely to occur in the case ofoperations taking place near the coast or in more closed waters, such asat tanker terminals or the like. When operations and installations inmore rough waters are concerned, e.g. far out at sea, buoy 7 as a rulewill be located well immerged under the sea surface. This is per se avery favorable situation for the buoy and the whole system, since thebuoy when located deep in the water is less subjected to influence fromwind and waves occurring at the sea surface. It is also an importanteffect of buoy 7 that under substantially all conditions this willmaintain anchoring line part 6 tensioned upwards from anchor 3, so thatno part of the anchoring line will be lying on the seabed 1.

There may also be cases where this buoy device comprises more than oneindividual buoy, but still so arranged that there is provided arelatively limited deflection portion more or less at the middle of thetotal anchoring line. The main purpose of such a buoy or buoy device isto provide for a relatively concentrated buoyancy in the anchoring line,which results in a soft or flexible behaviour of the whole mooringsystem, with reduced dynamic load effects.

In addition to the pure mooring function being explained above, such asystem can also comprise fluid transfer between the anchor means 3 andthe ship 10, such as loading thereof with hydrocarbons. Thus in FIG. 1there is shown a relatively flexible hose 9 being extended up to the bowportion of the ship 10, which is there provided with suitable connectionmeans, that may very well be combined with the mooring means 11. Suchmeans can be of designs being known per se. At a lower portion of hose 9there are shown buoyant elements 9A, which in this case are provided ina number of three, but can of course vary in number and dimensionsdepending on the desired shape of hose 9. A primary purpose of buoyantelements 9A is to secure that the lower portion of hose 9 is generallyalways elevated from seabed 1. It is a great advantage that hose 9 runsthrough the water well underneath anchoring line 6, 8, as illustrated inFIG. 1. Thereby any contact between the two main parts of the system isavoided, in particular so that hose 9 will not be damaged by any part ofanchoring line 6, 8.

Fluid transfer as mentioned above especially for loading a tanker, butalso possibly for unloading, is more particularly the subject matter ofthe simultaneously filed international patent application PCT/NO96/00202(our ref. INT6152L).

FIGS. 2 and 3 in more detail show a possible and preferred design of theanchor 3 with associated equipment, in particular a swivel device 5 atthe top of anchor 3. According to the invention this preferably has theform of a suction anchor, which can be of a design as known per se, andadapted to penetrate into loose masses underneath the actual seabed 1 inorder to obtain a strong anchoring effect. In the example shown in FIGS.2 and 3 the suction anchor 3 thus has a downwardly open cylindricalshape.

Centrally on top of anchor 3 there is shown a fixed carrier member 13which supports the actual swivel device 5. This has an upper connectionmember 19 with a pipe bend to which the lower end of hose 9 isconnected, e.g. by a flange connection. The lower swivel part 18 servesfor the attachment of two line parts 6A and 6B as shown more in detailin FIG. 3. Line parts or portions 6A and 6B constitute the lower end ofa so-called crowfoot having an apex at 6C (FIG. 1) so that the crowfootas a whole has the shape of a preferably isosceles triangle the baseline of which is formed by an arm structure 15A, 15B. This iscantilevered to each side from the lower swivel member 18 and is adaptedto be rotated together with the swivel part about the central axis ofthe complete anchor and swivel means. Arms 15A and 15B have a commonhorizontal axis 15C and line portions 6A and 6B respectively, areconnected to the outer ends of arms 15A and 15B so as to be pivotableabout the axis 15C. An important purpose of arms 15A and 15B is toprovide for a sufficient torque for the swivel movement about thecentral, vertical axis, depending upon the direction of the mooringforce from the ship 10 through the anchoring line 6, 8. Swivel members18 and 19 are united with respect to rotation.

In the arrangment described above in addition to rotation about avertical axis, there is the possibility also of pivoting or articulationabout a horizontal axis, namely axis 15C. Instead of a more or lessflexible crowfoot as mentioned, there can also be provided a more rigid,yoke-like design being incorporated in the anchor means as a whole. Bothin the case of a crowfoot and in the case of a rigid yoke conventionalattachment means or methods can be employed for the lower ends of theanchoring lines. Here there may also be the question of a relativelypermanent attachment or a connection that can be relatively easilylosened, that can e.g. be manipulated by means of an robot operatedvehicle. Such a possibility of detachable fastening consists in a deviceof the type “chain stopper”, which can be self-locking and otherwise canallow for manipulation or operation as known per se.

As seen in particular from FIG. 2 hose 9 has a direction outwards andupwards from swivel means 5 at a smaller angle in relation to thehorizontal than anchoring line portion 6A. When besides hose 9 as shownin FIG. 3, runs out centrally between line portions 6A and 6B, there isminimal risk of damage to hose 9 by contact with any portion of theanchoring line.

As a possible, but not preferred alternative, there is indicated at 9X adirection of the hose directly upwards centrally from swivel means 5,which implies that such a hose somewhere higher up in the water willhave to cross or pass by the anchoring line 6, 8. This is usually a lessfavorable solution. Finally FIG. 2 shows a pipeline 14 connected forsupplying e.g. produced fluid, such as hydro-carbons, to the anchorinstallation 3, namely the stationary carrier member 13 thereof for theswivel means 5.

The system described here can e.g. be intended for operation at waterdepths from 150-300 meters. At a depth of e.g. 200 meters the two parts6 and 8 of the total anchoring line can typically be 160 meters and 200meters respectively, in a favourable practical embodiment.

Otherwise it is obvious that various modifications and variants can becontemplated within the framework of the invention. Thus when it isstated that anchor 3 is permanent, this does not mean e.g. that asuction anchor or a gravitation anchor must remain forever at the seabed1, upon being installed. As known even such relatively fixedinstallations at the seabed can be removed by suitable means andequipment. A permanent anchor device in this context means a morepermanent anchor than what is typically carried by a ship and can bethrown from this or hauled into the ship by means of its normal anchorcapstan.

A method of installation of an anchor device in the system as explainedabove, according to the invention with advantage can consist in that theanchor is suspended at the end of an anchor chain or wire belonging to agenerally regular anchor capstan or winch of the ship concerned, beingemployed for lowering the anchor to a predetermined point at the seabed.

In FIG. 1 there is illustrated an apex 6C of the crow-foot as alsoexplained with reference to FIGS. 2 and 3, but it is obvious that theposition of apex 6C can vary considerably, and possibly the apex can beadjacent to or on the buoyant body or buoy 7. In the case of anapproximate vertical direction of the hose (as shown at 9X) from swivelmeans 5 in FIG. 2, it can be expedient to let the hose cross or pass bythe anchoring line 6 between the two portions 6A and 6B thereof in thecrowfoot, at a portion higher up in the water. It is also possible tolet this crossing take place adjacent to the buoy 7 when the apex 6C iscorrespondingly located, whereby the hose in such case can also besuspended from the buoy at this location.

Instead of a crowfoot as a prolongation of the anchoring line, asdescribed above, the system described here with associated anchor meanscan also be provided with a yoke or similar structure as shown anddescribed in the above mentioned, simultaneous international patentapplication.

What is claimed is:
 1. A system for mooring ships, for operations inconnection with oil and gas activity, whereby a ship is equipped with amooring device and where there is included an anchor device located atthe seabed, as well as at least one anchoring line adapted to connectsaid anchor device to said mooring device on the ship, wherein saidanchor device is a permanent anchor device being provided with swiveldevice for said anchoring line, said swivel device having a rotationaxis; a buoyant body attached to a middle portion of the anchoring line,and adapted during anchoring to be normally immersed in the sea; and acrowfoot provided at a lower portion of the anchoring line and connectedto said swivel device, whereby said swivel device comprises twocantilevered arms having outer ends to which the anchoring lines of thecrowfoot are attached, said two cantilevered arms extendingsubstantially perpendicular to the rotation axis of the swivel device.2. The system of claim 1, wherein said anchoring lines of the crowfootare pivotable about an axis between the cantilevered arms.
 3. The systemof claim 1, wherein said anchor device comprises one from the groupconsisting of a suction anchor, gravitation anchor and pile anchor. 4.The system of claim 1, wherein said two cantilevered arms have a commonaxis.